Titanium oxide pigment production



Patentedy Apr. 20, 1948 JohnlLexvs- KeatsAnde-Tamesllinward etsn,Wilmngtgn Delindsignqrsltp E VP ri lilas ,caused sulipro:

,Suchuaslthat te*V tra, titanium Sul.,- ingpowere'nd texture ogetherwh1sa. s mpaul rang? tintof ing. .strengthloil absizrptioe. Dfw eMoreuspecfially, rthehinventionl, pertains v-t; dlited t stl'levprepv.,rationofV improve lq-frogn 05 C.` or above,1the

memsnavmethwww spetrumon X-ray analysis a OdsfOrthQr. preparati@ by.,rtin .tirldu heet treatment at 'rele Y-lwf elinetvt@ peratures a.sub'stranti'ellyQ redu` ed` 'o1l dis;

grteijformof precpitatedmi en anata beqailseofeth -e ee, x'

res s prsiiw w11 smeris@ substantially".

Drs, been: already prof are, astheir primary ,object @0n- Ofthesejanatse landri'tile,colitilr'is th most useful as.pilg`rrifeifitls.` "Aiiatase est refractive index; whileir'tilel' 'posss,esth highest refractive index, anfdjtherefor estfptentialhidiigppowler. `pi` tase is` derive-Clv froixtliehydrolysi ofi, titsullflte"y solutions; While rutile @may be` obta from the hydrolytirprecipita"y 'fof ftitanf d l Thesefwexqessively Y chloridesolutions.AWhenfjthq ashed p Ti02 hydrolysate orrawlp gment prec Althouglianaltashas K the lowest vreflratiye index,` 'Inost present-'day ommeri'alffi'z"metsare oharaete'rize'd 'by theft modiatin Y appears that moreeoonoinicaland bondige iai attractive procedures arezh/atilaiblelfrfthisty of pigment, since, Aasis tv'eIT- krifW11',Ltl'ivef'sultf solutions employed in itfsrlftri atcturf-S.v are loorro'sivetowards plant eqn" Ymer-lt and Vzt'rlafr ily obtain, d fromslflirio acid ttakf p'qn taniferousores, such as i'lnqlnite;y 'Rutile ,t5l' ductionr from Chloride solutiiis, 'n th; 'other hand', 'requiresanvgei'ct'rem dime" l K Y1 type of vdpe'ration in which 'in cise'carefulcontrols.lfaft'glll'ltimevl of and the additonallkconfside" solutionsare hgh'lyl'oorrfo Y cause' nsiderable 'darnegeto I therelatively'expensivey'opelfelt 1 uct (as contemplated in U; S. patent to,B1umen-qfeld 1,892,693) and then calcined at temperatures sufcient to completelyconvert the anatase to rutile, it will be found that during theconversion a rapid increase in size takes place and a value ofapproximately 300 millimicrons particle radius may ultimately exist in'the com-.w pletely converted product. From the Ystandpoint ofdevelopment of optimum hiding'power, op-l timum particle size is of alower order than 300 and preferably from about 14d-225 millimicrons.,Since optimum particle size is essential to opf timum hiding power andother necessary titanium oxide pigment properties, many processes havebeen proposed for improving unsatisfactory particle size. In general,these contemplate prolonged wet or dry grinding, disintegration orcentrifuging of the titanium oxide after its calcination anddevelopment. Similarly and for the same purpose, dispersionv of thecalcined pigment, followed by recovery for use of the finer fractionsthrough classification, has beenalso proposed. None, however, has provedcompletely effective or satisfactory. At'the most the grindingoperations have onlysucceeded in making very minor reductions, while thedispersionA methods have been elective only in separating the particlesof various sizes which may be bound together in aggregates, and not inreducing the size of the particles themselves.

A long-felt need has thus existed for a method which enables one toreadily produce titanium oxide pigment having optimum values in respectto particle size and `within a definite and prescribed optimum range,whereby such product inherently possesses optimum hiding power,l tintingstrength, texture and other essential pigment values; for a usefulprocess from which this type of product is readily obtainable withoutrecourse to the prolonged, unsatisfactory, costly, and timeconsuminggrinding and separatory operations which have characterized priortitanium oxide pigment manufacture; and for a process in whichconversion of anatase to the higher (rutile) modication can be `readilyeffected without resort to previously required high temperaturecalcinationsv and their attendant deleterious eii'ect upon essentialproperties of the'final pigment.

It is among the objects of this invention to overcome the foregoingandother disadvantages inherent in prior titanium oxide pigmentproduction and to provide novel and useful methods for attaining theseends. A particular object of the invention is to provide a highly usefulprocess for producing an improved anatase or rutile'titanium oxidepigment having improved durability and weather-resistantcharacteristics, as well as optimum values in respect to uniform,average particle size, hiding power, tinting strength, texture,`colorand brightness, etc. Special .objects of the invention include theprovision of av novel method for manufacturing an improved rutiletitanium oxide Vpigment inherently possessing these optimum values byconverting precipitated raw anatase pigment at relatively lowcalcination temperatures, e. g., well below those heretofore considerednecessary, in order to obviate the disadvantages attending such previoushigh temperature conversion methods; to provide a process by whichrutile pigments of satisfactory color and with particle size in therange required for high hiding power can be obtained from the rawpigments produced in the commercial manufacture of anatase T102pigments; and, 'since the optimum particle size for development of highhiding power in the higher refractive index, rutile, islower than theoptimum for anatase, to provide a method of reducing the size of thesecondary aggregates in the hydrolysis precipitate so that aftercalcination the finished pigment will be in the proper range of particlesize. Other objects and advantages will be apparent from theensuingdescription of our invention.

These and other j objects are attainable in our invention which embracesthe discovery that the ,l properties of a finished, calcined titaniumoxide pigment, especially the higher, rutile modification thereof, maybe considerably improved if the raw pigment precipitate or hydrolysatefrom whence said pigment is derived is subjected, prior to heattreatment or calcination to develop essential pigment properties, tosuitable particle size reduction or disintegration by mechanical orequivalent means of treatment, and that the subsequent calcination ofthe reduced or disintegrated raw pigment results in the right degree ofparticle size growth and provides a final pigment having an averageparticle size in the range most suitable for optimum hiding power,tinting strength,paint film color, and other essential anatase or rutilepigmentary properties. We have discovered further that if the anataseprecipitate or hydrolysate from a titanium sulfate solution comprisesthe raw pigment so subjected to reduction or disintegration, itsconversion to rutile upon subsequent calcination can be effected at alower temperature than that required for raw pigments not' so processed,and that the final TiOz pigment Vwill likewise exhibit optimum values inrespect' to particle size, hiding power, tinting strength, color,texture, etc.

In its broader aspects, therefore, the invention comprises subjecting aTiO2 raw pigment to mechanical or like disintegration treatment toreduce the raw pigment particle size and then subjecting thedisintegrated product to calcination to develop its essential pigmentproperties.

More specifically, the invention comprises subjecting raw pigmentanatase to suitable reducing or disintegrating treatment to reduce theparticle size thereof, and then calcining the resulting product todevelop its pigment properties.

In a preferred embodiment, the invention comprises preparing ourimproved rutile titanium oxide pigment by subjecting raw pigmentanatase, obtained from the hydrolysis of a titanium sulfate solution, tomechanical disintegration treatment to reduce the particle size of saidraw pig ment, and then calciningthe resulting product, in the presenceof a small amount of a titaniumoxygen compound rutile conversionpromoter.

VTheV invention will now be described as applied to Ycertain specificand preferred embodiments thereof in which raw pigment anatase, from`conventional titanium sulfate solution hydrolysis, is treated inaccordance with the invention and is thenicalcined to effect itsconversion to rutile. Obviously, the invention is not limited theretobut may be beneficially applied to the treatment of all types of 'IiOzraw pigments, whether in the anatase cig-,rutile crystaiiine form,whether derived from the neutralization or hydrolysis of other forms oftitanium salt solutions, such as theznitrate, chloride, oxalate, etc.,and-'whether the `:finalfpigment comprises anatase-ior` rutile, orconsists of a mixture of both crystalline forms.

Iii accordance with one specic adaptation, a relatively pure,previously-washed anatase precipitate, obtainedf'rom the hydrolysis'of atisulfatesolution in` accordance with, for instance, the procedures ofU. S. Reissue Patent 18,854, or U. s'. Reissue Patentl 18,790 while inrelatively wet or dry state and prior to heat treatment to develop itspigment properties, is subjected to mechanical milling, grinding,pulvei'izing or extrusion treatment to disintegrate or 'reduce itsparticle size and substantially reduce the size of its secondaryparticles or aggregates. Conveniently this may be accomplished byemploying conventional types of grinding, milling, pulverizing orextruding media designed to disintegrate solid material or particles.Among examples of such type of media may be mentioned mills of the ball,pebble, rotary hammer, ring-roll or edge-runner type, squirrelcagedi's'integrators, micronizers, micropulverizers, or various extruding orejecting apparatus designed to exert a relatively high fluid ormechanical pressure upon a body of the raw pigment being ejected. One ormore of the following modes o'f treatment may be resorted to foraccomplishings said disintegration: (1) wet milling, (2) dry milling,(3) dry milling followed by wet milling or vice versa, (4) ejection orextrusion under relatively high pressure of an aqueous slurry of the rawpigment through a relatively small opening or restricted orifice, and(5) passing an aqueous slurry of the precipitate through a colloid millof the cone type or in similar types of mills,

wherein a very small clearance vbetween the stator and rotor members ismaintained. Preferably, the disintegration treatment is suicientlysevere and prolonged as to substantially redu'c'e the lsize `of saidsecondary aggregates and insure, upon subsequent calcination of thereduced raw pigment, procurance of a final product which will exhibitoptimum particle size, hiding power and tinting strength Values.

Invdisintegrating the raw pigment by wet milling, an aqueous slurryofthe pigment may be conveniently 4ground in the milling device. Ininstances of pebble mill use, the raw pigment may be ground for a periodof from about,.say, 1 to 4 hours or up to as high -as 64 hours, asuitable Water-to-pigment ratio employed during such milling being about4 to 1. Alternatively, the pigment may be first dried (at temperaturesranging from about 50200 C.) and then ground d'ry in the mill, apreferred procedure in this type of operation comprising dry grindingthe pigment, adding the water and then grinding the resulting Wet slurryfor an additional period. Thus, the mill may be rst iilled with pebblesto the extent of, say, `about 25-50% of its volume, the dried rawpigment may be then added and-ground `for a period of from about 1 to 4hours. Thereafter, the mill may be opened, sulicient Water introducedtherein to provide a 'slurry'containing about 300 g. TiOz per liter, andtheresulting Wet slurry may be then ground for 'an vadditional 1-4 -hourperiod, or longer, if `desired.

In producing mechanical disintegration by high pressure ejectionorextrusion, an aqueous slurry ofthe raw pigment, 'containing about Y250g. IiOa per liter; may-.be-placedlin asuitable vessel orf containervcapable of" withstanding; relatively' oriceory valve-continuedY outlet,through which theicontents off' the vesselcan beVV forced under anygiven or desired pressure. Gaseous iiuid such as air, hydrogemsteam,etc., is forced into the-reservoir under pressureand they free space inthe vessel containing the' pigment is. brought to the same pressure asthe reservoir. The outlet valve is'thenslowly opened-or. cracked yso.that the aqueous slurryfis forced from the vessel or tube through thesmallest openingl possible. In lieuof extrudin-g the rawk pigment from avessel containing the same through a restrictive. oriiice by-means ofappliediuidfpressure, mechanical means, such asa piston, pressure rolleror similar'equipment may be employed? in lieu of the rgaseous pressurein'- order to forcibly remove or eject thepigment fromay container andeiect its-reduction. For example,- an` aqueous suspensionofprecipitatedeTiOz may be introducedinto a cylindrical drum-havingoneor more converging narrow throatsl or restricted discharge outlets,towards, against and through which the precipitate willv beforced4` by asuitably actuated piston or roller means likewise disposed Within thecontainer or drum.

After effecting desired reduction, the disintegratedv product, ifvobtainedirom wetv milling, may be suitably dewateredf and then calcinedto develop essential Ti'OzA pigment properties and substantiallycompletely convert the same to rutile, if the raw` pigment undertreatment comprises ana'tase. Inins'tances Where the raw pigment is drymilled, calci-nation treatment can be effected directly *followingmilling. Prior to such calcination, a small amount (about .5 to aboutd1% von theTiOz basis), and-in accordance with the teachings` ofBlumenfeld Patent No. 1,892,693-, of an alkali metall salt, particularlysodium 'andyor 'potassium sulfate, may be incorporat-'ed in thedisintegrated product and the resulting mixture I'then I-cal'cir-ied.The calcined product isy then subjected to the usual pigment iinishingoperations, afterv which it becomes adaptable for-usein all pigmentapplications, and in those fields Where Wlziifte pigments are employed,such as 'in interior-and exterior coating compositions, particularlypai-nts, enamels, lacquers, linoleum, and sixnilar products, and inpaper and rubber com-pounding, rayon or other articia'l silk-delusterin-g, andk in various other fields. Because of its .relativelyhigh vhiding power and pigment density, our fini-shed lpigment,especially if in the rutile form, will be found to be particularly'useful wherever formulations are required-which employ `relatively lowpigment volumes. p

In a preferred adaptation of our invention and to obtain Ila rutile:pigment having exceptionally high hiding power andtintingcstrength asanal product, we .conveniently mix' with the :raw -anatase pigment,before or after the contemplated disintegration of 'said raw Epigment bymilling treatment, a .relatively small ramount of la Ti02 seeding 'or'catalyzing agent designed to promote anatase conversion to rutile`during icalcination. vlllseful lseeding lagents for the purpose.comprise precipitated :raw rutile y'prepared from Y:titanium chloride4solutions for fsomea form of Vhydrous TiO2 which, when :calcined alone,fconverts Lato .rutile at which the anatase raw pigment converts whenvcalcined alone; .Thus,- specifically useful types of rutile promoterscomprise precipitated 'raw rutile obtained from thehydrolysis of atitanium chloride solution, in accordance with the procedures of U. S.Patent 2,062,133, or the hydrplysates obtained according to thedisclosure of the copending and now abandoned applications of John L.Keats, et al., Serial No. 283,052, iiled July 6, 1939 (nowcontinuation-impart application Serial No. 500,148, filed August26,1943), and Ser. No. 283,051, led July 6, 1939. The amount of rutilepromoter to be used is, as already stated, relatively minor and mayrange, for instance, from substantially 1-25 parts of rutile per 100parts of anatase, a highly useful concentration being from about 2-10parts of rutile foreach 100 parts of anatase present. After admixture ofthe raw pigment anatase and rutile promoter has been effected, theresulting product is milled or otherwise disintegrated to reduce theparticle sizethereofand the ground mixture may be then dried at about100 C. and -furtherground in a pebble mill for a period of about 2-4hours. Sufiicient Water is then introduced into the mill to provide aTiOz slurry of about 275 g./1. and the grinding continued for anotherperiod of about one hour. 'In such instances, the temperature ofconversion to rutileon subsequent calcination will be found to be muchlower than that of the seeded anatase after wet grinding alone.

To `obtain a pigment having optimum values in respect vto colorandbrlghtness as well as improved softness and uniformitycharacteristics, we prefer, after incorporating the rutile promoter orseeding agent in the reduced anatase raw pigment, or after an anataseraw pigment-rutile promoting agent mixture has been suitablydisintegrated, and priorto calcination, to mix with .said material asmall amount of a mixture comprising an alkali metal salt, particularlysodium :and/or potassium Vsulfate and certain polyvalent imetalcompounds adapted to yield white, insolu- V'.ble oxides (in accordancewith the disclosure of the copendng application of James H; Peterson,

LSer. No. 426,249, led January 9, 1942, now Patent No. 2,369,246), andthen calcine the treated, disintegrated raw pigment at rtemperaturesranging from substantially 750C. to not in excess of about 1000 C. (andpreferably from 850 C. to 975 C.) todevelop essential pigment propertiesand substantially completely convert the anatase to rutile. The calcinedproduct may be then sub- `jected to the usual pigment finishingoperations,

after which it is adapted for use in all types of pigment application,especially those mentioned.

To a more complete understanding of the invention, the followingspecific examples are given, each being given by Way of illustration butnot in limitation of our invention:

Example IA This example relates to wet grinding anatase raw pigment in apebble mill.

Anatase raw pigment prepared by following the procedures Aof U. S.Reissue Patent 18,854 to Blumenfeld was slurried with water to give asuspension containing 250 grams T102 per liter. The slurry was thenground for 16 hours in a pebble mill containing pebbles 1/2 to inch.-The apparent Vvolume of the balls in the mill was 35% of mill capacityand the volume of the balls plus charge was 55% of. mill capacity. Theparticle size 'of the ground raw 'pigment was substantially lower thanthat of the unground product as is shown by the following data:

- Y Average radius Unground raw pigment 284 mlllimicrons Raw pigmentground 16 hrs 231 millimicrons o 1 o irvergge Grinding of Raw PigmentTenc Pfugt Cagnuesd Product Mllimcrons None 1,025 100 313 Wet ground 16hours l, 000 100 215 Example II Anatase raw pigment prepared by acommercial process following the procedure disclosed in U. S. Re. 18,854to J. Blumenfeld was seeded with raw rutile in the ratio of one partrutile by weight to 10 parts anatase by weight. The raw rutile wasprepared by hydrolysis of titanium chloride solution by a proceduresimilar to that disclosed in U. S. 2,062,133 to P. Kubelka and J. Srbek.The two components were mixed as thick slulries and the concentration ofthe mixture adjusted to 250 grams TiOa per liter. The slurry was groundfor a period of 64 hours in a pebble mill containing pebbles ofdiameters 1/2 to 5A; inch under the same conditions of mill charge asare outlined in Example I. The ground prod-uct was characterized byparticle size lower than the original anatase raw pigment as is shown bythe following data:

. Average radius Seeded anatase raw pigment-not ground 284 lmillimicronsSeeded anatase ray pigment-Wet ground 64 hrs 213 millimicrons Thereduction in particle size of the raw pigment caused by grinding carriedthrough to the calcined pigment and gave a finished product of smallerparticle size and considerable higher tinting strength than was obtainedby calcination of the unground control raw pigment. The ground productsubstantially completely converted to rutile at a lower temperature thanthe unground product.

Average Grinding of Seeded Raw Calc. Per Cent Radius Tinting1 PigmentTemp. Rutle Calcined Strengt Product one l, 025 313 156 Wet YGround 64hrs 1,000 100 210 176 Y Example III 9 the chargewas-rotatedlinafstandard pebblemill frame,rior aeperiodn of.'16 liours'.Thehiiished rutile :pigmentiobtainedbysaltl treatmentewith small-amounts olf-sodiumr and'. potassium sulfate andecalcinationAof-thesand,-"ground ,seeded 'raw 5 pigmenty wasmcharacterizedfby; lowerparticle l,size andsubstantially.'y higher tintig? strength' than theproduct obtainedfby similar, treatmentv*of the. i-rawe pigment. Withoutgrindinge. The averagei radius-` oft-` the. sandground product e after10 calcination ata-temperature of975`3" C(Y tol give substantiallycomplete-conversion to rutile was 199mi1ii'microns. and thetiming-,strength was 187.

Example4v IV- Anatase raw pigment prepared-asl described in-Example I4mixedwithrutile'raw pigment inrthefratioofk-one.part rutile to 10-parts, anatase by Weight. Particle size measurementsby the 20spectraltransmissiom method. indicated that the averageradiusofthe-mixt'uredwas 331 millinicronse-A The mixture of anatase and rutileraw pigments Hwas` dried lat 11.05 andthen ground e dry ina-pebblemillin whichtheballs oceupiedrabout 25 196:millimicrons.Productsobtained by calcina- 30 tion ofA4 similarA seeded AanataseAraw-- pigments Witl-ioutV the dry grinding, step. preceding thecalcination have rshownA Aaverage radii.. of l above 300 millimicron'seEample V Ahata'se raw 'pigrierit from' the same lot" as tlat des'cbdl i!Example "I was' mixed with Arau/1riitil prepared by tli'hydrolvsis oftitanium tetrachloride in the? ratio^offone-`part `rutile to lilparts'anatas'ebylweight'IiOz.- The raw pig-y mentsr` were mixed'ias vwetslurries and themixed suspension ltere'dand -the cake `Vdried; at 1 10'C.

mill'j containing 'porcelain 'balls equal. in Volume t 25% of thecapacity' of 'the mill; The product wasgroundtfor a period lof threehours the dryifim after Whicliw'ater equal' 'tothree times tlie Weight'of fthe vdried'pigiinent "was added; and 5 mixture,andftne'produ'ctdried"at 110-and then 55 grind butwas'treatedtle samein' `ally other respectsl` are* givenA for comparison:

l0 Eample VI Anatase raw pigmentfpreparedf asdescribed in Example VI.was mixed. with. rutile raw pigment prepared from hydrolysis of titaniumtetrachloride solutionl to giv-efi a suspension containing 275VAgrams-"IiOe per llit'er and-containing one `part rutileffr1l).'parts.anatase.A The e suspension` was placed inside a vertical-isilver-lined steel bomb Atube capable oi withstanding,highpressures. Thebomb tubeiwas connectedatfthe upper end with a tube leadin'gftoiahydrogenv compressor and was equipped with an outlet1 valve at 'thelbottom. Hydrogen was forced into the free' space'within the bombaboveithe suspension to give a pressure Within the bomb" of'7500'pounds` per square inch. A Valve on the 'leadi'intube Was thenclosed. 'Ihereui'flety valve -attthe bottom of .the bomb was therropenedvery, slightly y and# the slurry Was ejected-,throught thesmal-Lopeningrv The vslurry vbefore ejection occupiedffhalf "ofi thespace in` the bomb` The pressure#y dur-ingY ej ectionwv thereforedeereased'lfrom- 7500-poundsf-to about half vthis amount.l Theejectedslurry'was then vplaced in thee bomb again-and the-processVrepeated until `5J ejections were completedr on.tl1e.material. The ejectiorr treatment 1effectedsubstantial reduction in1z artole-` sizeottherawfpigment. and this reduction 'w'ase reflected einlower-.particle size and higher .-etinting-V-fstrength` ofthe ,productafter -calcinati'om This is-showr1 bythe-following tabulationtV AverageRadius I of Product 35" l l AverageRadius CalcinedEiectionof-Seedelifltaw Pigmentf SeededaRaw' tofGve Pigment CompleteConversion to Rutile 401. Mz'lZzmz'cz-m'my i None: l..- 238 Above 270msluccessive" 'Ejections' at 7500 l pounds/su;i1:1. .l 152 181 Example'VII. f

Anatase' raw pigment, prepared" as described in Example .I, was mixedWith` rutileV raw pio'- ment preparedby hydrolysis' of titaniumtetrachloride to givea vsuspension'containing 210 v,grams 0 Tier per,liter and eenteining'rutiie endenateee in theratio-V of V1":"10'.. Themixed-slurry was placed in. av.bomb capable ofiwitlistanding. highpressures.

, Thebmb was mounted vertically, the' upper f endbeing connected `withahydrogen compressor, N"and a. storage reservoirwas betweenthecompressonandthe bomb. v An outlet valve Was provide'dat the lower end-of the bomb. After lplacingtheecharge in thebomb the compressor wasoperated` until the pressure Within the bomb and 60 reservoirwas15j000fpoundsper square inch.' The line connecting .thecompressor andreservoir was then. closed but. the, linevconnec'ting the bombwitlfitl'ie.ree'zser'v/"oir"l was leftopen; The-valve at thebottom-ofthe bombw'asfopenedvery slightly and `the gas. Withintl'e bomb andreservoir forced the .mixedjslurry.through the small orifice underthe.constantpressureof 15,000pounds per square inch. .Y Substantialreduction. in`v particle size of "theraw pigment. was caused by the sin-Stren'eth 70 gle ejection at this'pressure. The ejectedV productaftercal'cinationlat 'a temperature which effected Isubstanti'allycomplete "conversion to ruti1e-v was.` also. subtantially lower in'particle size than. ay similar product which .had not received fthepressure ejection but which hedieeen treated the same in all other ways.the following tabulation:

Data are given in Anatase raw pigment prepared by hydrolysis of titaniumsulfate solution was mixed with rutile raw pigment prepared by thehydrolysis of titanium chloride solution to give an aqueous suspensioncontaining 250 grams TiOz per liter and containing rutile and anatase inthe ratio by weight of 1:.10. The slurry was ground in a Travis Dispermill. The ground slurry was recirculated through the mill for a periodof 30 minutes. Substantial reduction in particle size of the seeded rawpigment was effected by the colloid mill grinding. The product obtainedby calcination of the ground mixture at a temperature giving completeconversion to rutile was also characterized by relatively low particlesize. Average radius for the ground product before calcination was 212millimicrons compared with 262 millimicrons for the unground product.The average radius of the ground product after calcination at 975 was225 millimcrons and its tinting strength was 1'72 compared with 150 forcommercial anatase.

The tinting strength and hiding power values given above were determinedby the methods described in U. S. Patent 2,213,542, dated September 3,1940.

The particle sizevalues given were obtained by a turbidimetric methodcalibrated by the ultracentrifuge, the latter being described in U. S.Patent 2,062,134`

By the terms raw pigment particles or secondary particles We refer tothe units into which either freshly precipitated or dried raw T102pigment can be dispersedby moderate milling in a mortar with someappropriate vehicle such as glycerol. Eachof these raw pigment particlesis made up of a large number of smaller or "primary particles. 'Ihesecondary particles can be dispersed to give a suspension of the primaryparticles by neutralizing the residual sulfuric acid in the raw pigment,washing out the soluble sulfates and addition of a monobasic acid suchas hydrochloric .or nitric acid to the desulfated raw pigment. ByV theterms disintegrated or mechanically reduced rawpigment we refer to theproduct which results from our contemplated milling or grindingtreatment of a raw pigment TiOz hydrolysate or precipitate. "Finishedpigment particles or calcined pigment particles comprise the units intowhich the calcined pigment can be dispersed by mulling with anappropriate vehicle such as an allryd resin or glycerol.

While the invention has been described as applied to certain preferredembodiments thereof in which specific types of raw pigment precipitates,seeding agents, amounts, ratios, temperatures, volumes, etc., have beenemployed, it is not to be considered as limited thereto. As alreadystated, the invention broadly contemplates the manufacture of improvedtitanium oxide pigments, whether inthe anatase or rutile crystallinemodification or comprising mixtures of both modiiications. For thepurpose, it contemplates the treatment of all types of raw TiOz pigmentprecipitates or hydrolysates, and especially those resulting from U. S.Reissue Patents 18,854 or 18,- 790, or from U, S. Patent 1,851,487. Itis especially effective for obtaining an improved rutile titanium oxidepigment through conversion of disintegrated raw anatase by calcinationin the presence of a rutile seeding agent or promoter. It is especiallyuseful for obtaining an improved rutile pigment through conversion at arelatively low calcination temperature of a disintegrated form ofpurified raw pigment anatase precipitated from the hydrolysis of atitanium sulfate solution and for converting reduced, seeded anatase rawpigment having particles or secondary aggregates of average radius in arange such that when the raw pigment is calcined in the presence of asmall amount of an alkali metal salt, at a temperature adapted to givecomplete conversion to rutile, the calcined product is characterized byhaving optimum hiding power and tinting strength characteristics.

Although we refer to the use of specific TiOz slurry concentrationsduring wet grinding, ranging from substantially 250-300 g./l., these aremerely preferred. In most practical applications, the use of slurriescontaining from about 100-350 g. TiOz per liter will be found to beadvantageously useful in the invention. Although the use of higher andlower TiOz concentrations is contemplated, it will be found that whenthe concentration ranges above about 450 g. TiOz per liter, relativelyviscous slurries result, and hence such concentrations are notrecommended if optimum results are desired in the invention. Similarly,although concentrations below 100 g./l. are contemplated as employableherein, the greater amount of mill capacity which such concentrationsrequire renders their use in the invention less .practical andsatisfactory. Hence, such lower concentrations are not recommended, ifit is desired to derive optimum benets in the invention.

Likewise, though we prefer to utilize calcination temperatures rangingfrom substantially 850 to 975 C., and not in excess of 1000 C, to insurethe procurance of optimum benefits and advantages under the inventionand production of a final pigment which is essentially all rutile andpossesses optimum values for tinting strength, hiding power and particlesize, the invention is not limited to the indicated lower limit ofspecied temperature. The use of any particular calcination temperaturewill obviously depend upon the nature of the TiOz precipitate undertreatment and the particular properties and extent of rutilecrystallinity which it is desired that the final product shall exhibit.In general, it will be found that temperatures ranging fromsubstantially '750 to 1000 C. may be usefully and effectively employedin the invention.

Although we have described our invention as particularly adaptable forthe manufacture of straight or unextended forms of anatase or rutiletitanium oxide pigments, it will be obvious that extended forms of suchpigments may be readily produced herein. Thus, various inorganicextenders, such as barium or calcium sulfate, calcium carbonate ormagnesium silicate, may be suitably blended with or precipitated uponthe IQz pigment during the course of its manuencaisse grinding. and 2-ejection.; treatments' inr...accordance with our inventionrtopbtainfarawfpgymentwliiclr;

when subsequently" calcined-,Jiexhibitsroptimum particle sizelvalues,'-isrobviouslyavariazblez Thus, the time off.rdisintegratingsrtreatment: may, :vary fromzay relatively.fshortrtimedsaw about l honra; tota relatively:longatirneg(say.lilhours While longer. grinds? arel Vmore eiectivef inz'redncingxparrticle z size,I they r may', iniv some instances', .-befedissadvantageous. if yi camiedzoutxfor protracte ee'- riods, especially;fromwthestandpoi-nt .oflcone taminatingthe-ra-w-orundeveloped'pigmentiiwitlr the material presentrrinvthe`balls: forspebblessem: ployed in theY giinding-.=-;-`

In. reducing partiel er. sizeibiy :dry-:grinding oreby dry: grinding.fol-lowedaby fwet .grindinggsprefere ably,v theraw.'pigmenthisifirstztdriedy 'remove the majorportion@ofi'thefmoisturevfromittlreawetl cake, drying:temperatures;l.ranginge'from about 50e-200 C...being.f.useful.1 forxthepurposer.Brodie ucts dried :y atV the s: lower:: temperatures: -(beiow about. 125C.)v appearfto undergo greaterrreduce tionk in lparti-cle sizeduring:subsequent .dryegrinde ing .thanl those.. driedffat".-higheri'temperaturess Hence, the;.use^ oiathesef loiwerf. dryingD:temper-ar tures 1 is .preferredu` 'I'he Igrindingrzcvcle. lwille biefound' :tozbezrelatively:importantdnzthefdiyi pluswet.grindingz'proicessrs .Tlfetimeeoftdry grinding isfpreferablyfromaahoutal-fiihoursi'gbiumnaateyary overwiderflimitspes.iggpirom:ahoutxrl fminnte'sezto about- 24 hours.Il Dry;grinding:forxmorersextensive periods; may resulta-inproducts;lackingzirndesired uniformity oirpartiizle.sizeffand:lhence'sewe pref-er to avoid such. extendedzfdrgy; grinding periods. Thevwet. grind .iolflbwingA` thxdr'y grind is preferably made at aconvenient slurry consistency and the periodofwet'- grinding'may varyfrom a very short periodfsay, from.v yminutestosseveral hours, 1-3 hourshaving beeny found.to..comprise a most practical and effective lengthottime for the wet grinding "or'milling operation;

In reducing particle sizezbwejection of aqueous lsuspensions.through asmalli'orice under-:high

fluid pressure, ,the,pressnre-funden whichtth'e'isuspension isejectedWill be foundtc be important and critical: Pressures` of'therordenfofif^2000 pounds'pensquare inch are-useful vrhen; only moderate;reduction in partielewisize isidesired. To eiiect greater reduction in.raw pigment'iparticle size, higher pressures must be resorted to.Pressures of the order of 10,000- -l5,000 pounds per square inch havebeen found most advantageous and are therefore preferred. In genera-l,several ejections at one pressure will usually accomplish the sameresult as one ejection at a somewhat higher pressure. Thus, 5 ejectionsat 7500 pounds pressure produce reduction in particle size aboutequivalent to one ejection at 12,000 pounds. However, the greatesteiciency is obtained by a single ejection at the required pressure sincethe percentage reduction in size for several successive ejectionsdecreases with each ejection. In its broader aspects, the use in theinvention of fluid pressures within a range of from, say, 1000 to 20,000pounds per square inch, is contemplated.

While the invention has been illustratively describedgin-ts z.applicatiom tuv :so-calledi .bat chg. as distinguishedn'romfcontinuousg typesofifmille ing :orextrudingz apparatus; ,itiwillibexunderstood thatrra-w"pigmentdisintegration mayube vadvan-e tageouslyi:eiected';v in.- such; continuous f types i. of media, wherebymillingiagrindingx or ejection' of tlrenprecipitated rawpigmentfthrough. a: restrictedoriiicemayzbieieiected fbya continuous :pumpf ingfor=-.recyclingfoperation'; Vas :distinguished from theribatchstype;ois;the:;examples..-. Obviously,vv such continuous `:typesfoffoperations are usually; more feasible land;economical.inrhandlinglarge rquantities oir-materials.; Also; theiraw pigmentunder treatmentmay: be:v disintegrated: under room oratmospheric:-Vtemperaturesfasiwell, as atrielevated ternperaturesf.`orsrrsuperatmospheric pressure.

As .williheievident fromcth-e. foregoing; one Ycan readilyifI :obtain:by.;` our', inventionx anA improved forrnzof :titanium oxide? pigment;and. especially rutile; .that .the lattenzcan': bei I produced by:conversion-off. .anatase'and-.vwithout iresorting: tok theundesirablyfrhigh: conversion41 .z temperatures 'requiredfheretoioreandLW-hichiincur; objectionable sintering .whichztadversely affects .thecolor, uniformity, 1r particle`v size; texture, tinting s. strengthandfhidin'g-.p owe-renfe` the fina-l product.: Due tosuchzgexcessivelyahigh temperatures; prior rutile converted"productenare-:characteristically icoarse andgritty:inznature:andipriorito -use it has :b een necessaryto rsubjectthemst'o; expensive and pro,- lo'ngedf: grinding. treatmentsiinf-an :ieffort .to reduce.. at-'g Aleast :1to1 some" degree; the z size of;these aggregates;` andzharda particlest,Z This: has not proyedientirely;satisfactory orreiiectivei By 'our inventiomthesefzdisadvantages: are.:remedied and ai directprocessl visf-.provided ion obtaining apigmentzwhich. inherently; possesses; optimum par.-ticle-size;-and:1tinting strength :values4 is ofdesirably-iy soit:smooth texture, randa: exhibits excelient.;hiding; power:andl color:characteristics withxrelativel-y, complete` freedom. from-:objectionable'sicoarseapartieles:or:V sintered s. aggregates.l

As willilbe urtherr evident; theainventionzfaffordsri a; inovelymet-hod;for-'reflecting :fa substantial reduction; in the @ai/orage; particlev`size-fy of .a calcined'ititanium oxide pigmentiwhetherythat pig.-mentzis.: insthef. anatase. :or:I rutilemodiiication or compriseseamixturegv-of both ,crystalline f'forms. For example. whereas: prior.calcined, converted Vrutile;pigmentsV have .r been; characterized; by:an

average .radii i ofi the vorder. 0178001 millimiCrQnsor above; those;obtained" in ourinvention-are characteristicallyflower;in size,;wellibelow. 300 millimicrons; andzmore ynea'ri 2.001 millimierons.Consequently, .'ounpigments-.being:within an: optimum particle; sizef:range. inherentlyexhibit: maximum hiding Vpower,ttintingstrength; andother.r essen.- tial pigment properties. Further, the invention isespecially adaptable for processing anatase raw pigment or seededanatase raw pigment to provide an undeveloped pigment product havingparticles or secondary aggregates of average radius in a range such thatwhen the disintegrated raw pigment is calcined in the presence of asmall amount of an alkali metal salt at a temperature which givesrelatively complete conversion to rutile, the calcined product ischaracterized by average radius in the range of about -225 millimicrons.

We claim as our invention:

1. A process for iproducing an improved rutile titanium oxide pigmentcomprising subjecting, prior to heat treatment to develop its pigmentproperties anatase raw pigment titanium. oxide to mechanicaldisintegration treatment to reduce 15 the particle size thereof, andthen calcining the disintegrated raw pigment in the presence of atitanium-oxygen compound rutile conversion promoter in order to developessential pigmentary properties in said raw pigment and substantiallyconvert the same to rutile.

2. A process for producing an improved rutile titanium oxide pigmentcomprising subjecting, prior to heat treatment to develop its pigmentproperties precipitated anatase raw pigment to mechanical disintegrationtreatment to reduce the particle size thereof, and thereafter calciningthe reduced product in the presence of a small amount of a rutileconversion promoter comprising precipitated raw pigment rutile, in orderto develop essential pigment properties and convert said anatase toSubstantially rutile.

3. A process for obtaining an improved rutile titanium oxide pigmentexhibiting optimum values in respect to particle size and tintingstrength which comprises subjecting, prior to heat treatment to developits pigment properties a relatively pure, anatase raw TiOz pigmenthydrolysate obtained from the hydrolysis of a titanium sulfate solutionto mechanical disintegration treatment in order to reduce the particlesize oi said raw pigment and then calcining the reduced product at atemperature: not in excess of sub-1 stantially 1000 C. in the presenceof a minor amount of precipitated raw rutile pigment.

4. A process for obtaining an improved rutile titanium oxide pigment byconversion of anatase which comprises subjecting, prior to heattreatment to develop its pigment properties a mixture of anatase rawpigment and a minor amount oi raw pigment rutile to mechanicaldisintegration treatment in order to reduce the particle size of saidraw .pigment mixture, and thereafter calcining the mixture of reducedraw pigment at temperatures ranging from substantially 750-1000" C.

5. A process for obtaining an improved rutile titanium oxide pigmentthrough conversion of .precipitated raw anatase pigment which comprisessubjecting, prior to heat treatment to develop its pigment properties araw pigment anatase hydrolysate obtained from the hydrolysis of atitanium sulfate solution to mechanical disintegration treatment inorder to reduce the particle size thereof and thereafter calcining thedisintegrated product at a temperature ranging from substantiallyF150-1000" C. in the presence of from about 1 to 25 parts oi raw pigmentrutile per 100 parts of disintegrated raw pigment anatase.

6. A process for obtaining an improved, calcined, rutile titanium oxidepigment having a particle size characterized by an average radius in therange of about 140-225 millimicrons, which comprises subjecting, priorto heat treatment to develop its pigment properties a raw pigmentanatase hydrolysate from the hydrolysis of a titanium sulfate solutionto mechanical disintegration treatment, in order to reduce the particlesize thereof, and thereafter calcining the disintegrated product at atemperature ranging from substantially 750 to 1000 C. in the presence ofa small lamount of an alkali metal salt and from about 1-25 parts of rawpigment rutile per 100 parts of disintegrated raw pigment anatase.

7. A process for obtaining an improved, calcined, rutile-convertedtitanium oxide pigment having a particle size average radius in therange of from about 140-225 millimicrons, comprising subjecting, priorto heat treatment to develop its pigment properties raw pigment anatasefrom the hydrolysis of a titanium sulfate solution to mechanicaldisintegration treatment to reduce the particle size thereof, andthereafter calcining the resulting disintegrated raw pigment at atemperature ranging from substantially 750 to about 1000 C. in thepresence of from about .5 to 1% of sodium sulfate and from about 2-10.parts of raw pigment rutile per parts of the disintegrated raw pigmentanatase present.

8. A process for obtaining an improved, calcined, rutile-convertedtitanium oxide pigment having a particle size average radius in therange of from about -225 millimicrons, comprising subjecting, prior toheat treatment to develop its pigment properties raw pigment anatasefrom the hydrolysis of a titanium sulfate solution to mechanicaldisintegration treatment to reduce the particle size thereof, andthereafter calcining the resulting disintegrated raw pigment at atemperature ranging from substantially 750 to about 1000 C. in thepresence ci from about .5 to 1% of potassium sulfate and from about 2-10parts of raw pigment rutile per 100 parts of the disintegrated rawpigment anatase present.

JOHN LEWIS KEATS. JAMES HOWARD PETERSON.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED) STATES PATENTS Number Name Date 1,922,328 Rhodes Aug. 15, 19331,995,580 Rockstrok et al Mar. 26, 1935 2,034,923 Schmidt Mar. 24, 19362,062,134 Kubelka Nov. 24, 1936 2,133,941 Allan Oct. 25, 1938 2,273,431Booge Feb. 17, 1942 2,290,539 Cole July 31, 1942

